1. Field of the Invention
The present invention relates to a method for fabricating a display panel used in a monitor for an audiovisual apparatus such as a television set, an office automation apparatus, or the like; and a method for cutting a substrate which can be preferably used in the method for fabricating a display panel.
2. Description of the Related Art
In recent years, liquid crystal display devices (hereinafter, referred to as "LCDs") have been used in various fields. Advantages for using an LCD over other conventional display devices include their light weight due in part to their recent significant size reduction in a depth direction, i.e., realization of an extremely thin thickness; their ability to be easily installed in a small space; their low power consumption; and their ability to readily realize a full-color display. The LCD generally includes a liquid crystal display panel, a driving circuit, a backlight, and the like.
During a process for fabricating a liquid crystal display panel, a step of cutting a mother substrate, on which electrical lines are formed by a known process, into a predetermined size is needed irrespective of the type of the liquid crystal display panel. For example, in a liquid crystal display panel having electrical lines provided in a X-Y matrix shape, all of the electrical lines are short-circuited up to some point in the process for fabricating the liquid crystal display panel in order to avoid defects caused by static electricity. Therefore, in order to insulate individual electrical lines from each other, a step for cutting a substrate is also needed.
In accordance with known LCD fabricating process, there are generally two different times when such substrate cutting steps are performed: (i) after fabricating a panel by attaching together a pair of mother substrates with electrical lines provided thereon, the substrate is cut to a predetermined size; and (ii) after a pair of mother substrates.with electrical lines provided thereon are cut to a predetermined size, respectively, the thus-obtained substrates are attached to each other to produce a panel.
Known methods for cutting a substrate includes: (1) a diamond scribing method as shown in FIG. 10A, (2) a laser scribing method as shown in FIG. 10B, (3) a diamond blade dicing method as shown in FIG. 10C, and (4) a laser cleavage-cutting method as shown in FIG. 10D. Particularly in the fabrication of a liquid crystal display panel, the diamond scribing method shown in FIG. 10A is often employed from the standpoint of productivity.
Along with the onset of the information age, display devices used as a monitor for audiovisual apparatus such as a television set, an office automation apparatus, or the like came have been required to have high definition and a large screen. In order to provide a large screen in display devices such as a cathode ray tube (CRT) based display, a LCD, a plasma display, an electro luminescence display (hereinafter, referred to as an "EL display"), or a light-emitting diode (LED) display, developments have been made and display devices having such large screens have been put into practice. At the same time, since realization of such a large screen has lead to increases in weight, size, and power consumption of the display device, it has been ongoing challenge to reduce the weight and thickness of the display device, and to realize a lower power consumption in the display device.
Among the display devices described above, the LCD can satisfy the aforementioned needs since the LCD is light-weight due in part to its significantly thin size which also allows it to be installed in a small space. In addition, the LCD can satisfy the aforementioned needs due to its low power consumption while still being able to readily realize a full-color display device. Moreover, the LCD is suitable for use in a display device with a large screen such as a large size monitor, a wall-mounted display device, or the like. Therefore, as to the realization of a large screen, a greater expectation is placed on the LCD as compared to the other display devices.
When the screen size of a liquid crystal display panel is increased, however, a defect rate rapidly increases due to the breakage of signal lines, pixel defects, and the like in the process of fabricating the liquid crystal display panel. In addition, the price of the liquid crystal display panel also increases. Therefore, in order to solve these problems, Japanese Laid-open Utility Model Publication No. 60-191029, for example, discloses a liquid crystal display panel composed of one large substrate which is fabricated by connecting a plurality of small substrates along their side surfaces so as to compose at least one of a pair of substrates constituting the liquid crystal display panel.
FIG. 11A is a plan view showing the liquid crystal display panel disclosed in Japanese Laid-open Utility Model Publication No. 60-191029. FIG. 11B shows a cross-sectional view showing the liquid crystal display panel cut along line I--I in FIG. 11A. Herein, four active matrix substrates are connected in the manner shown in FIG. 11A (two substrates in a row.times.two substrates in a column) so as to make a large substrate. A liquid crystal display panel with a large screen is fabricated by attaching the thus-obtained large substrate with other appropriate substrates (i.e., a color filter substrate) having a liquid crystal layer interposed therebetween.
In an active matrix type liquid crystal display panel, it is, in general, extremely difficult to fabricate a large substrate on which a micro-active element may be provided in each pixel (i.e., an active matrix substrate) while still allowing for a high yield of LCD displays. Therefore, in terms of productivity, the fabrication process which will now be described is considered to be more effective.
More particularly, in this fabrication process, a plurality of small substrates are fabricated and connected with one another at the side surfaces thereof to make a large active matrix substrate. Thereafter, a large substrate (i.e., a counter substrate) with a color filter provided thereon is attached to the active matrix substrate so as to fabricate a panel.
According to such a liquid crystal display panel with a large screen, in order to make stitching between substrates less noticeable, it is necessary to make a connecting region between the substrates as small as possible by accurately cutting a connecting surface between the active matrix substrates. Therefore, in fabricating such a liquid crystal display panel, from the standpoint of processing accuracy, the diamond blade dicing method is often employed in order to accurately cut the connecting surface.
According to the diamond scribing method as depicted in FIG. 10A, among the aforementioned methods for cutting a substrate, a cutting projected line is marked off using a diamond needle, and a substrate is divided by applying a mechanical bending stress. With this method, it is possible to easily perform a cutting process. Therefore, the use of the diamond scribing method allows for a high rate of manufacture. However, the diamond scribing method has problems such as the creation of small particle like chips resulting from breaking the substrate (hereinafter, referred to as "broken chips"), poor alignment accuracy of about several hundreds micrometers, and the like.
According to the laser scribing method as described in FIG. 10B, after the surface of a material is melted and evaporated by irradiating a cutting projected line with a laser beam so as to form a slot, the substrate is cut by applying a mechanical bending stress thereto. Although the laser scribing method can realize a non-contact and high-speed processing, there exist problems such as the scattering of the melted and evaporated substrate as particles, thereby adhering to the surface of the substrate, the generation of broken chips, and the like.
According to the diamond blade dicing method as depicted in FIG. 10C, a grinding processing is performed by high speed rotation of a blade to which abrasive diamond grains are adhered and applying the rotating blade to the substrate. The diamond blade dicing method produces excellent processing accuracy. However, this method requires water for grinding and washing, and has problems such as the generation of grinding chips, and the like.
The laser cleavage-cutting method as depicted in FIG. 10D utilizes a thermal stress which is generated inside the material when irradiated with a laser beam. By moving such a heat source, cleavage develops. The laser cleavage-cutting method does not generate broken chips, particles, grinding chips, or the like, and requires no water for grinding or washing. Recently, the laser cleavage-cutting method has been applied for the cutting of various types of substrates. For example, JOURNAL OF THE JAPAN SOCIETY FOR PRECISION ENGINEERING (p. 196, Vol. 60, No. 2, 1994) briefly describes such an application.
In the case where the laser cleavage-cutting method is used for cutting a substrate for a liquid crystal display panel, however, problems as described hereinafter will arise.
Electrical lines, insulation film, and the like often exist along a cutting line of a substrate for a general liquid crystal display panel. For example, in the case of a passive matrix type liquid crystal display panel, electrical lines formed of transparent conductive films exist along a cutting line. In the case of the active-matrix type liquid crystal display device, on the other hand, electrical lines formed of metal films, insulation film, and the like exist on a cutting line. This is because of the fabrication process described above in which during the forming of these electrical lines on a mother substrate, all electrical lines are short-circuited in the peripheral region of the substrate in order to avoid defects caused by static electricity, and each electrical line is isolated from each other by cutting off the peripheral region in the step for cutting the substrate. However, in the case where the laser cleavage-cutting method is used on a substrate having such electrical lines, insulation film, and the like, a laser beam may be reflected depending on the existence of the electrical lines, and the amount of absorption of a laser beam varies depending on the existence of insulation film. As a result, a thermal stress which is generated when irradiated with a laser beam fluctuates, thereby not being able to smoothly cut the substrate with high accuracy.
Moreover, in the case where the laser cleavage-cutting method is used for cutting the connecting surface between the active matrix substrates for the aforementioned liquid crystal display panel with a large screen, problems as described below will arise.
Upon cutting such an active matrix substrate, it is necessary to perform cutting in the vicinity of a display pixel in a highly accurate manner in order to make the stitching between the substrates less noticeable by making the connecting region as small as possible. For such a purpose, it is required to dispose a pixel electrode, a semiconductor element (for example, a thin-film transistor (hereinafter, referred to as a "TFT")), and the like at positions about 100 .mu.m to about 500 .mu.m away from the edge of the connecting surface, although the distance varies depending on the size of the liquid crystal display panel with a large screen and the resolution thereof.
Since the aforementioned laser cleavage-cutting method is a method utilizing a thermal stress which is generated when irradiated laser beam is absorbed in the vicinity of the surface of the substrate, the vicinity of the laser irradiated spot is more likely to be affected by heat conduction. As a result, any semiconductor element (i.e., TFT), which is disposed in a position close to the connecting surface is heated and adverse effects such as a change in characteristics are thus generated. Moreover, when a semiconductor element (i.e., TFT) is directly irradiated with a laser beam, adverse effects such as optical deterioration may occur.